From 8e37ee25985eaf28a6d4a74d3fe4d2c937c12a36 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C5=8Dshin?= <git@wholezero.org>
Date: Sat, 15 Jun 2024 10:45:52 -0700
Subject: [PATCH] ctl::string cleanup (#1215)
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We now fully initialize a ctl::string’s memory, so that it is always set
to a well-defined value, thus making it always safe to memcpy out of it.
This incidentally makes our string::swap function legal, which it wasn’t
before. This also saves us a store in string::reserve.

Now that we have made both big_string and small_string POD, I believe it
is safe to elide the launder calls, and have done so, thus cleaning up a
lot of the blob-related code.

I also got rid of set_big_capacity and replaced it with a set_big_string
that leaves us in a well-defined state afterwards. This function also is
able to be somewhat simpler; rather than delicate bit-twiddling, it just
reaches straight into blob and rewrites it wholesale.

Overall, this shaves about 1–2ns off of most benchmarks, and adds 1ns to
only one of them - creating a string from a char *.
---
 ctl/string.cc |  7 ++-----
 ctl/string.h  | 40 +++++++++++++++++-----------------------
 2 files changed, 19 insertions(+), 28 deletions(-)

diff --git a/ctl/string.cc b/ctl/string.cc
index f5184786f..503ade6a8 100644
--- a/ctl/string.cc
+++ b/ctl/string.cc
@@ -91,16 +91,13 @@ string::reserve(size_t c2) noexcept
     if (!isbig()) {
         if (!(p2 = (char*)malloc(c2)))
             __builtin_trap();
-        memcpy(p2, data(), size());
-        p2[size()] = 0;
+        memcpy(p2, data(), __::string_size);
     } else {
         if (!(p2 = (char*)realloc(big()->p, c2)))
             __builtin_trap();
     }
     std::atomic_signal_fence(std::memory_order_seq_cst);
-    set_big_capacity(c2);
-    big()->n = n;
-    big()->p = p2;
+    set_big_string(p2, n, c2);
 }
 
 void
diff --git a/ctl/string.h b/ctl/string.h
index 5315a0a45..736067721 100644
--- a/ctl/string.h
+++ b/ctl/string.h
@@ -81,28 +81,23 @@ class string
 
     string() noexcept
     {
-        set_small_size(0);
-#if 0
-        small()->buf[0] = 0;
-#endif
+        __builtin_memset(blob, 0, sizeof(size_t) * 2);
+        // equivalent to set_small_size(0) but also zeroes memory
+        *(((size_t *)blob) + 2) = __::sso_max << (sizeof(size_t) - 1) * 8;
     }
 
     void swap(string& s) noexcept
     {
         char tmp[__::string_size];
-        __builtin_memcpy(tmp, __builtin_launder(blob), sizeof(tmp));
-        __builtin_memcpy(
-          __builtin_launder(blob), __builtin_launder(s.blob), sizeof(tmp));
-        __builtin_memcpy(__builtin_launder(s.blob), tmp, sizeof(tmp));
+        __builtin_memcpy(tmp, blob, __::string_size);
+        __builtin_memcpy(blob, s.blob, __::string_size);
+        __builtin_memcpy(s.blob, tmp, __::string_size);
     }
 
     string(string&& s) noexcept
     {
-        __builtin_memcpy(blob, __builtin_launder(s.blob), sizeof(blob));
+        __builtin_memcpy(blob, s.blob, __::string_size);
         s.set_small_size(0);
-#if 0
-        s.small()->buf[0] = 0;
-#endif
     }
 
     void clear() noexcept
@@ -288,52 +283,51 @@ class string
   private:
     inline bool isbig() const noexcept
     {
-        return *(__builtin_launder(blob) + __::sso_max) & 0x80;
+        return *(blob + __::sso_max) & 0x80;
     }
 
     inline void set_small_size(size_t size) noexcept
     {
         if (size > __::sso_max)
             __builtin_trap();
-        *(__builtin_launder(blob) + __::sso_max) = (__::sso_max - size);
+        *(blob + __::sso_max) = (__::sso_max - size);
     }
 
-    inline void set_big_capacity(size_t c2) noexcept
+    inline void set_big_string(char *p, size_t n, size_t c2) noexcept
     {
         if (c2 > __::big_mask)
             __builtin_trap();
-        *(__builtin_launder(blob) + __::sso_max) = 0x80;
-        big()->c &= ~__::big_mask;
-        big()->c |= c2;
+        *(char **)blob = p;
+        *(((size_t *)blob) + 1) = n;
+        *(((size_t *)blob) + 2) = c2 | ~__::big_mask;
     }
 
     inline __::small_string* small() noexcept
     {
         if (isbig())
             __builtin_trap();
-        return __builtin_launder(reinterpret_cast<__::small_string*>(blob));
+        return reinterpret_cast<__::small_string*>(blob);
     }
 
     inline const __::small_string* small() const noexcept
     {
         if (isbig())
             __builtin_trap();
-        return __builtin_launder(
-          reinterpret_cast<const __::small_string*>(blob));
+        return reinterpret_cast<const __::small_string*>(blob);
     }
 
     inline __::big_string* big() noexcept
     {
         if (!isbig())
             __builtin_trap();
-        return __builtin_launder(reinterpret_cast<__::big_string*>(blob));
+        return reinterpret_cast<__::big_string*>(blob);
     }
 
     inline const __::big_string* big() const noexcept
     {
         if (!isbig())
             __builtin_trap();
-        return __builtin_launder(reinterpret_cast<const __::big_string*>(blob));
+        return reinterpret_cast<const __::big_string*>(blob);
     }
 
     friend string strcat(const string_view, const string_view);